1. Field of the Invention
The present invention relates to plasma processing apparatuses, and more particularly, to a plasma processing apparatus that forms a thin film on the surface of an object of interest or that etches the surface of an object taking advantage of plasma.
2. Description of the Background Art
FIG. 9 is a schematic sectional view of a conventional plasma processing apparatus disclosed in, for example, Japanese Patent Laying-Open No. 2-9452. Referring to FIG. 9, the conventional plasma apparatus includes a vacuum vessel 101, a first electrode 103 on which an object 102 to be processed is placed, and a second electrode 104 arranged opposite to first electrode 103.
Etching gas is introduced through a gas inlet 105 into vacuum vessel 1 and exhausted through an exhaust port 106. A high frequency power source 107 is connected to first electrode 103 via a matching circuit 108. A permanent magnet 109 is arranged at the atmosphere side of second electrode 104. A cooling mechanism 110 is connected to first electrode 103. In FIG. 9, E indicates the electric field and B is the component of the magnetic field induced by magnet 109, parallel to first electrode 103.
The operation of the plasma processing apparatus of the above structure will be described hereinafter. When etching gas is introduced into the plasma chamber of vacuum vessel 101 from gas inlet 105, plasma is generated between first and second electrodes 103 and 104 by the high frequency power applied to first electrode 103.
The apparatus shown in FIG. 9 is directed to achieve high electron density even at a low pressure by magnetron discharge. The apparatus of FIG. 9 is set so that the magnetic flux density at the surface of first electrode 103 is approximately 200 G.
At the sheath region (the region where plasma is in contact with first electrode 103), the charged particles (electrons and ions) drift in the direction of E.times.B while moving cycloidally under the influence of the sheath electric field and magnetic field.
As a result, the probability of collision between an electron and a neutron (molecule, atom) increases to promote ionization. Accordingly, plasma of high density is generated even at a low pressure to achieve a high etching rate. In this case, plasma loss is reduced by the magnetic field caused by permanent magnet 109. Therefore, the high density plasma is maintained to allow etching of object 102 of interest.
It is now necessary to generate uniform plasma over a large area for the purpose of processing objects of large diameter such as 8 inches or 10 inches in size. However, the magnetic flux density in the lateral direction (parallel between electrodes) at the surface of second electrode 104 of the above-described plasma processing apparatus with the arrangement of a single permanent magnet is low at the center and becomes higher uniformly in the radial direction as shown in FIG. 10(B). The overall magnetic flux density is not uniform. It is therefore difficult to form a magnetic field of uniform intensity in the proximity of the object to be processed.
It is not easy to generate plasma uniformly despite its homogeneous action by diffusion. FIG. 10(A) shows a permanent magnet of 200 mm in diameter and 50 mm in height with the surface magnetic flux density entirely uniform at 3 kG. FIG. 10(B) is a graph of the magnetic field distribution in the lateral direction at the surface of second electrode 104 remote from the permanent magnet of FIG. 10(A) by 35 mm. The magnetic field intensity B .perp. (G) in the lateral direction is plotted along the ordinate, and the distance r(mm) from the center is plotted along the abscissa.
The magnetic field distribution at the surface of the object to be processed placed on first electrode 103 is also not uniform. Since the movement of a charged particle is greatly affected by the magnetic field distribution, the flux of the incident charged particles at the surface of the object to be processed is also not uniform, reflecting the nonuniform magnetic field distribution. As a result, the distribution of the charge density is disturbed at the surface of the object to be processed to damage the device.
In the event that a plurality of permanent magnets are used, the magnetic field distribution will be nonuniform similar to the above case with a single magnet if the permanent magnets are arranged so that adjacent magnets have the same polarity. Therefore, the uniformity of the plasma will not be sufficient even if the homogeneous action by plasma diffusion is taken into account.
The aforementioned Japanese Patent Laying-Open No. 2-9452 also discloses arrangement of a plurality of rod-like permanent magnets with the polarity between adjacent magnets being the opposite, as shown in the sectional view of FIG. 11(A). When the polarity is altered alternately, the distribution in the radial direction of the lateral magnetic flux density B .perp. at the surface of second electrode 104 is indicated by the waveform of FIG. 12(B) according to the arrangement of permanent magnets 109 of FIG. 12(A).
It is appreciated from FIGS. 12(A) and 12(B) that the position of the peak can be controlled by altering the distance between the magnets although B .perp. is not uniform radially. Homogeneity can be achieved by generating the plasma in such a magnetic field coordination since the plasma is spread by diffusion even to the region where the magnetic field is weak. Because of reduction in loss in contrast to the case where there is no magnet, uniform plasma of high density can be obtained.
In the parallel arrangement of a plurality of rod-like permanent magnets 109 as shown in FIG. 11(A), magnetic fields B.sub.1 and B.sub.2 are generated as shown in FIG. 11(B). At region (A) in the proximity of the object to be processed, the plasma drifts in the direction piercing the plane of the drawing sheet by the E.times.B drift caused by electric field E and magnetic field B.sub.1. At the region of (B), plasma drifts in the opposite direction by electric field E and magnetic field B.sub.2 to become locally dense.
Focusing on the movement of the charged particles at the sheath portion at the surface of second electrode 104, the direction of drift (indicated by arrows) differs for every pair of adjacent permanent magnets 109 due to the E.times.B drift as shown in FIG. 13. The region of the shaded area indicated by X in the drawing has high density due to the high plasma density portion corresponding to the direction of the drift. As a result, nonuniformity occurs in the plasma density with the parallel arrangement. This means that uniformity in the etching rate is degraded. This is a critical problem in the parallel arrangement of permanent magnets.
FIG. 14 shows a schematic structure of another conventional plasma processing apparatus having the plasma generation chamber and the processing chamber divided. Such a plasma processing apparatus is disclosed in Japanese Patent Laying-Open No. 51-88182, for example. Referring to FIG. 14, a processing chamber 121 is evacuated by a diffusion pump 132 via a main valve 131 and an auxiliary rotary pump 133. A plasma generation chamber 122 is provided above processing chamber 121. Counter electrodes 118 and 119 are connected to ground at plasma generation chamber 122. Counter electrode 119 with a plurality of holes 20 is provided as a partition wall to divide plasma generation chamber 122 and processing chamber 121. A raw material gas cylinder 134 is connected to a gas conduit 115.
The operation of the plasma processing apparatus of the above structure will be described hereinafter. The etching gas introduced into plasma generation chamber 122 through gas conduit 115 passes through processing chamber 122 to be output by a vacuum pump. Difference in pressure between plasma generation chamber 122 and processing chamber 121 is generated by the conductance of holes 120 provided between plasma generation chamber 122 and processing chamber 121.
According to specific figures disclosed in the prior art, the pressure of plasma generation chamber 122 is maintained at 1.about.5.times.10.sup.-1 Torr and the pressure of processing chamber 121 is maintained at not more than 1.times.10.sup.-3 Torr under the conditions of seven holes each having a diameter of 0.1-0.8 mm at the effective evacuation rate of 1000 L/sec. of the exhaust system and the flow rate of 50-100 cc/min. of the raw material gas.
By supplying a high frequency power to counter electrodes 118 and 119 by a high frequency power source 117, plasma is generated in plasma generation chamber 122. The plasma passes through holes 120 to etch object 102 placed on a table 126 in processing chamber 121.
In the plasma processing apparatus of the above structure, the plasma generated at plasma generation chamber 122 by the parallel-plate high frequency discharge had the density of 5.times.10.sup.8 (particles/cm.sup.3) to 5.times.10.sup.9 (particles/cm.sup.3). The processing speed of object 102 is relatively proportional to the density of incident plasma towards object 102. Limitation in the generated plasma density prevents the object of interest to be processed at high speed since plasma of high density cannot be introduced into processing chamber 121. Also, the pressure in plasma generation chamber 122 maintained by the parallel-plate high frequency discharge is approximately 0.1 Torr. Thus, there was a problem that the object cannot be processed at an atmosphere under a high vacuum.
In the conventional plasma processing apparatus having a single magnet arranged as in FIG. 9, the magnetic flux density increases uniformly from the center radially. Therefore, a uniform magnetic field distribution cannot be provided. Therefore, nonuniformity is exhibited in plasma density.
In the plasma processing apparatus having a plurality of magnets arranged in parallel with alternate polarity between adjacent magnets as shown in FIGS. 11(A) and 11(B), the drifting direction differs for every pair of adjacent magnets. An area of high plasma density is generated in the drifting direction to result in nonuniform plasma density. There was a problem that an object of a large area could not be etched uniformly.
In the plasma processing apparatus having separate plasma generation and processing chambers as shown in FIG. 14, the density of the plasma generated in the plasma generation chamber is low. Therefore, plasma of high density cannot be introduced into the processing chamber. As a result, the process cannot be carried out at high speed. There was also a problem that the object cannot be processed at the atmosphere under a high vacuum if the plasma density is increased.